The figure schematically illustrates a prototype general-purpose, microprocessor-based sensor and control system in which power, data, and control signals are transmitted via optical fibers. This system serves as an experimental model for the further development of networks of remote sensors connected to monitoring and controlling base stations via optical fibers.
In one important class of potential applications, networks of the type envisioned would be embedded in structures to monitor strains and vibrations. Another important class of potential applications would take advantage of the electrical isolation afforded by optical fibers; networks of sensors could be deployed to monitor voltages and currents and/or to command the operation of switches at critical locations along high-voltage power lines. Some aspects of systems that might eventually be developed were discussed in "Fiber-Optic Distribution of Pulsed Power to Multiple Sensors" (NPO-19420), NASA Tech Briefs, Vol. 20, No. 5 (May 1996), page 18a, and "Data Protocol for Optically Powered Sensors" (NPO-19421), NASA Tech Briefs, Vol. 21, No. 6 (June 1997), page 6a.
In the present system, power for the remote station (sensor node) is supplied by a small laser of the type used in compact-disc players. The laser light is transmitted via an optical fiber to the remote station, where a photovoltaic cell converts the light to electric power. By modulating the laser light, the base station can transmit commands to the remote station; this amounts to an optical analogue of the established power-line-carrier technique, in which an electric power line is used to carry small ancillary control or data signals superimposed on the main power signal.
At the remote station, the outputs of sensors are multiplexed, digitized, and preprocessed, and the resulting data are converted to pulses of current supplied to a light-emitting diode (LED). The resulting pulses of light from the LED are transmitted along a second optical fiber to the base station, where a PIN (positive/intrinsic/negative) photodiode converts the light pulses to electrical data pulses. The returned sensor data are sent to a personal computer for further processing and display.
The operations of the base and remote stations are controlled by microprocessors. The microprocessor in the remote station is of a low-power type and is programmed with software that puts the remote-station circuitry into a low-power "sleep mode" most of the time to conserve energy. When needed, this circuitry can be "awakened" by transmitting a command via the power-supply optical fiber.
This work was done by Harold Kirkham, Larry A. Bergman, Shannon P. Jackson, Alan R. Johnston, and Duncan Liu of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP)free on-line at www.techbriefs.com under the Electronic Systems category,or circle no. 172on the TSP Order Card in this issue to receive a copy by mail ($5 charge).